1. Field of the Invention
The present invention relates to a gateway apparatus arranged between a circuit switched network and an internet protocol network (IP network) to repeat data transmitted/received between circuit switched networks through the IP network.
2. Description of the Prior Art
As one communication technology of sound (audio) data, the sound data is transmitted through an IP network such as the Internet or an intranet. This technology is called voice over IP (VoIP) connection. Application software or a system according to the VoIP connection is called an internet telephony (IT).
An internet telephony gateway apparatus (to be referred to as a “gateway” hereinafter), having a gateway function which performs protocol conversion between a circuit switched network such as a telephone network and an IP network, for realizing two-way communication between the circuit switched network and the IP network. An internet telephony system (IT system) for repeating data between circuit switched networks by using the gateway are realized. The IT system rapidly spread because communication charge of the IT system cheaper than communication charge of a conventional system using only the telephone network.
In the IT system described above, when communication of sound data is performed between circuit switched networks, the gateways set a communication channel for forming a connection in which corresponds to a call between the circuit switched networks, on an IP network. The sound data is transferred by using the set communication channel. The communication channel is set such that gateways on an call-out side and an call-in side execute a procedure (to be referred to as an “H. 323 procedure” hereinafter) based on H. 323 protocol. The H. 323 protocol is communication protocol corresponding to VoIP recommended by recommended by ITU-T (International telecommunication Union-Telecommunication).
The H. 323 procedure comprises (1) a basic call connection procedure based on H. 225 protocol, (2) a capability exchange procedure between gateway apparatuses based on H. 245 protocol, and (3) a connection procedure of an sound communication channel (sound channel). Upon completion of these three procedures, transmission/reception (sound communication) of sound data between terminal devices accommodated in circuit switched networks is started.
The capability exchange procedure based on the H. 245 protocol is performed as shown in FIG. 5. First, a gateway on a call-out side (call-out side gateway) transmits a message (H. 245 Terminal Capability Set (call-out side)) including the capability information of the gateway apparatus (step S1). The capability information includes, for example, a CODEC form (including compression/expansion form and transmission rate).
When a gateway on a call-in side (call-in side gateway) receives a message including the capability information of the call-out side gateway, after the call-in side gateway transmits a massage of an acknowledge (H. 245 Remainal Capability Set Ack) (step S2), and the call-in side gateway transmits a message (H. 245 Terminal Capability Set (call-in side)) including the capability information of the call-in side gateway (step S3).
When the call-out side gateway receives the message including the capability information of the call-in side gateway, the call-out side gateway transmits an acknowledge message (H. 245 Remainal Capability Set Ack) of the message (step S4). In this manner, the capability information is exchanged between the gateways.
Thereafter, when the call-in side gateway receives the acknowledge message from the call-out side gateway, the call-in gateway transmits a connection message (H. 245 Open Logical Channel) of a control channel related to H. 245 (step S5).
When the call-out side gateway receives the connection message, the call-out side gateway transmits an acknowledge message (H. 245 Open Logical Channel Ack) of the connection message (step S6). In this manner, a connection channel related to H. 245 in a downward direction (call-out side←call-in side direction) is set between the gateways.
Thereafter, when the call-in side gateway receives the acknowledge message from the call-out side gateway, the call-in side gateway compares the capability information received from the call-out side gateway with the capability information of the call-in side gateway, extracts capability information which can be mutually executed by the call-in side gateway and the call-out side gateway, and selects one the extracted capability information. For example, when a plurality of CODEC forms which can be mutually executed by the call-out side gateway and the call-in side gateway are extracted, one of the plurality of CODEC forms is selected. Thereafter, the call-in side gateway transmits a message (H. 245 Request Mode (call-out side←call-in side)) including the selected capability information (selection result) (step S7).
When the call-out side gateway receives the message including the selection result, after the call-out side gateway transmits an acknowledge message (H. 245 Request Mode Ack (call-out side←call-in side)) of the message (step S8), the call-out side gateway transmits a connection message (H. 245 Open Logical Channel) of a control channel related to H. 245 (step S9).
When the call-in side gateway receives the connection message, the call-in side gateway transmits an acknowledge message (H. 245 Open Logical Channel Ack) of the connection message (step S10). In this manner, a connection channel related to H. 245 in an upward direction (call-out side→call-in side direction) is set between the gateways.
Thereafter, when the call-out side gateway receives the acknowledge message from the call-in side gateway, like the call-in side gateway, the call-out side gateway compares the capability information received from the call-in side gateway with the capability information of the call-out side gateway, extracts capability information which can be mutually executed by the call-in side gateway and the call-out side gateway, and selects one the extracted capability information. Thereafter, the call-out side gateway transmits a message (H. 245 Request Mode (call-out side→call-in side)) including the selection result (step S11).
When the call-in side gateway receives the message including the selection result, the call-in side gateway transmits an acknowledge message (H. 245 Request Mode Ack (call-out side→call-in side)) of the message (step S12). The acknowledge message is received by the call-out side gateway.
With the procedures described above, the call-out side gateway and the call-in side gateway exchange the capabilities and select one of capabilities (CODEC form and the like) which can be mutually executed by the call-out side gateway and the call-in side gateway to allocate the selected capability to sound channels in the upward direction and the downward direction.
In this manner, after each gateway sets a CODEC form or the like, sound channels in the upward direction and the downward direction are set. When each gateway receives sound data, the gateway expands and compresses the received sound data according to the set CODEC form, and transmits the sound data at a set transmission rate.
In the capability exchange procedure described above, the gateway respectively select CODEC forms from capabilities which can be selected. More specifically, when each gateway selects a CODEC form related to an sound channel set on an IP network, the gateway does not considers the CODEC form of sound data received from the upstream side (circuit switched network) of the gateway. When each gateway selects a CODEC form related to sound data to be transmitted to a circuit switched network, the gateway does not consider a CODEC form related to a communication channel on the upstream side (IP network) of the gateway.
For this reason, when each gateway selects a CODEC form different from the CODEC form on the upstream side, as shown in FIG. 6, if sound data which is compressed by the circuit switched network (shown as a private branch exchange (PBX)) on the input side is received, the corresponding gateway expands the compressed data, compresses the expanded sound data in a different compression form, and transmits the compressed sound data to the IP network. A gateway which receives the compressed sound data from the IP network expands the compressed sound data, compresses the expanded sound data in a different compression form, and transmits the compressed sound data to a circuit switched network (PBX).
Since each gateway independently decides the CODEC form of sound data, if the CODEC form on the reception and transmission sides of the gateway coincide with each other, as in the above description, the gateway performs expansion/compression of sound data.
The fact described above is one of factors that extend time required to transmit sound data between terminal devices connected to circuit switched networks. A case wherein a gateway apparatus performs an unnecessary expansion/compression process may occur.